1. Field of the Invention
The invention relates to the field of audio compressors.
2. Prior Art
Compressors adaptively adjust their gain to automatically reduce the dynamic range of an audio signal applied to their inputs. Traditionally, compressors use either feedback or feedforward control. In a feedback compressor, the gain-controlled output of the compressor feeds a full-wave rectifier or comparator with threshold level, either of which responds to any signal exceeding the "threshold level of compression." If the envelope of the audio at the compressor's output exceeds the threshold level, the rectifier or comparator sends a signal into a smoothing or integrating circuit, which outputs a signal that causes gain reduction. If the smoothing circuit is a true integrator, it will continue to charge until the output of the compressor no longer exceeds the threshold level of compression, at which point charging will stop. (The time required for this charging to fully occur is usually called the "attack time" of the compressor.)
If the input audio level decreases, a release circuit permits the integrator to discharge, increasing the gain until the compressor's output is once again at threshold level, or until the compressor has achieved its maximum quiescent gain. The time constant of this discharge is usually called the "release time."
In a feedforward compressor, the input of the compressor is connected in parallel with the input of an envelope detector, such as an average or r.m.s. detector. The output of the envelope detector is applied to a thresholding circuit with the output of this circuit controlling the gain of the compressor. Usually, the output of the envelope detector is proportional to the logarithm of the envelope ("linear in dB"), and the gain (in dB) of the gain determining circuit (usually a voltage-controlled amplifier or "VCA") is directly proportional to the signal applied to its gain-controlled port. By choosing the constant of proportionality between the output of the thresholding circuit and the input of the VCA, one can arbitrarily set the compression ratio (the slope of the output level versus input level characteristic, in dB).
In any compressor, the dynamic characteristics of the envelope detector or integrator (such as attack and release time constants) must be matched to the psychoacoustic requirements of the ear to minimize audible artifacts. Because in a feedforward compressor it is easy to process the output of the envelope detector without compromising the stability of the control loop, such designs have become very popular. However, the available envelope detectors (such as average or r.m.s. detectors) have tended to introduce obvious artifacts.
Some designs of feedback compressors have a dynamic response smoother than most commercial feedforward compressors. A significant amount of this smoothness results from the unique dynamic characteristics created by the feedback control. However, these feedback designs have two important limitations. First, it is difficult to smooth the output of the integrator well enough to prevent some audible distortion. This distortion is caused by modulation effects when the gain-control signal is multiplied by the audio. Second, it is inconvenient to create a compression ratio of less than infinity because the feedback always attempts to hold the output level of the compressor equal to the threshold level of compression. (See British Broadcasting Corporation Engineering Division, "The dynamic characteristics of limiters of sound programme circuits," Research Report No. EL-5 (1967), serial number 1967/13.)